The M3XTRACT program extracts a variable or a selected subset of variables from a CMAQ file for a specified time period and a specified area and writes them to a separate CMAQ file. Optionally, the variables can be renamed during the process. Before running this program, or any other of the utilities, one is allowed to assign logical names to the physical file names of both files, according to IO/API conventions, using the operation
setenv <lname> <pname>
where lname equals the logical file name (e.g., INFILE), and
pname equals the physical file name (e.g., NC_195aa_cc3_g1)
The program will prompt the user for the logical names chosen for the input file and then extensively for the options to choose. The prompts will have default responses which can be accepted by hitting <RETURN>. The specification of starting date and time, target date and time, and duration for the run follow CMAQ date and time conventions.
This program is extremely useful in compressing large data files. For instance, an output concentration file containing 41 variables for a 16 × 23 × 26 domain occupies 226 Mbytes of disk space. The original concentration file can be saved to an archive while the analysis proceeds on a set of smaller files. Extracting just the ozone concentrations will result in a 5.5-Mbyte file. Further extracting only the surface layer ozone values will reduce the file size to 0.2 Mbytes. This utility is also useful for changing variable names within a CMAQ input/output file.